Methods for producing bio-oil from a feedstock involving (1) pyrolyzing the feedstock in an inert atmosphere in a reactor to produce bio-oil, bio-char and non-condensable gases; (2) recycling about 10 to about 99% of the non-condensable gases to the reactor to produce deoxygenated bio-oil; wherein the method is conducted in the absence of oxygen and wherein the method does not utilize externally added catalysts.
Fast pyrolysis has become the most promising method for production of liquid fuel intermediates from lignocellulosic biomass (Mohan, D. et al., Energy Fuels, 20: 848-889 (2006); Huber, G. W., et al., Chem Rev., 106: 4044-4098 (2006)). The pyrolysis process holds promise for utilization in small on-the-farm systems because of its smaller footprint and the logistical advantage of transporting dense liquids over bulky biomass (Wright, M., and R. C. Brown, Biofuels Bioprod. Bioref., 1: 191-200 (2007); Wright, M. et al., Biofuels Bioprod. Bioref., 2: 229-238 (2008)). However, it is well documented that biomass fast pyrolysis oils have compatibility issues with the current infrastructure, whether they are to be used for stationary boiler fuels or upgraded to hydrocarbon transportation fuels due to their high acidity and instability, problems mostly associated with high oxygen content. For that reason it has been the goal of many pyrolysis researchers to produce deoxygenated pyrolysis oils resulting in better characteristics for direct combustion and an easier path to “drop in” transportation fuels via various upgrading methods including a hydrotreating process.
To produce the desired deoxygenated fuel intermediates, many have focused on adding an oxygen rejecting catalyst to the pyrolysis process. Most of the reports on catalytic pyrolysis involve the use of solid acid catalysts such as zeolites to promote cracking type reactions (Mullen, C. A., et al., Energy Fuels, 25: 5444-5451 (2011); Mihalcik, D. A., et al., Eng. Chem. Res., 50: 13304-13312 (2011); Carlson, T. R., et al. ChemSusChem, 1: 397-400 (2008); Jae, T., et al., J. Catalysis, 279: 257-268 (2011); Cheng, Y. T., et al., Angew. Chem. Int. Ed., 51: 1387-1390 (2012); Williams, P. T. and N. Nugranad Energy, 25: 493-513 (2000)). The general mechanism by which these catalysts work are through protonation of oxygenates and generation of carbocations through dehydration. These reactions produce olefins which aromatize under the reaction conditions. However, the removal of hydrogen via these types of reactions from already hydrogen deficient feedstocks results in coke formation which reduces the carbon conversion to the liquid product and also deposits coke on the catalyst thereby deactivating it. Therefore reactor design for catalytic pyrolysis systems must provide for continual regeneration of catalysts which results in a more complex system than one for thermal only pyrolysis. These systems may require additional footprint, controls, expertise, and expense to run, and complicates the process of deploying an on-the-farm or mobile system of this type.
It is therefore desirable for the scale of interest to produce partially deoxygenated stable fuel intermediates without the use of limited lifetime catalysts.
We have found that when recycling volatile pyrolysis products are used as fluidizing gas and tuned to specific concentrations to provide a reducing reaction atmosphere, an autocatalytic effect can occur and partially deoxygenated pyrolysis oils are produced without the use of externally added catalysts.